After rearranging the equipment and reconnecting the infant to the ventilator, the flowmeter used during P-V measurements was used to measure Cdyn as tidal volume divided by the difference between end-inspiratory pause pressure and PEEP. e. Cdyn was obtained between points of date me zero airway flow. (Flow usually ceased also during end-inspiration-even before the end-inspiratory pause.)
FRC was measured with a computerized multiple breath washin-washout technique, using SFsix as tracer gas (18) . The method allows repeated measurements (each washin-washout sequence takes 1-2 min), without interfering with ventilator settings or the inspired oxygen concentration. Briefly, the system included a ventilator (Servo 900C), inspiratory and expiratory valves (360P0121, Neptune Research, Maplewood, NJ) placed in the patient circuit close to the Y-piece, and a mainstream SF6 infrared transducer-analyzer (19) with its cuvette placed just downstream the expiratory valve. Pressures(end-inspiratory pressure and PEEP) were measured via a pressure port in the Y-piece by the ventilator pressure transducer, which was moved into close contact with the Y-piece. 00) connected to differential pressure transducers (MP 45-1-871, Validyne Engineering Corp., Northbridge, CA). For an FRC measurement to be accepted the difference between inspired and expired tidal volumes had to be less than 10%. SF6 was washed in at a rate proportional to inspiratory flow, until a stable alveolar concentration of approximately 1.5% was achieved. Signals representing flows and SF6 concentration were fed into the computer, which calculated FRC as the volume of SF6 washed out divided by the alveolar concentration at the end of washin. The apparatus volume between the tip of the SF6 dispensing catheter and the cuvette windows (1.5 mL) was subtracted.
Clinically used PEEP varied between 2 and 4 cm H2O, and FRC at this end-expiratory pressure (FRCp) was measured at least twice in each infant and the mean taken. FRC at ZEEP (FRCz) was measured in 11 infants (5 in the group with severe RDS and all in the air-ventilated group) by switching to ZEEP 5-10 breaths before the start of washout. z was measured only once. Five infants were judged too unstable for measurements during ZEEP (indicated by footnote d inTable 1). In these, FRCp was used to estimate FRCz, assuming that FRCp – FRCz equaled the volume change between the corresponding pressures in the P-V curve. The regression equation between calculated and measured FRCz in the 11 infants where FRCz was measured was: calculated FRCz = -1.4 mL + 1.09 ? actual FRCz (r = 0.91). The difference between measured and calculated FRCz was -5 to +7 mL (median 1 mL).
Inhomogeneity of ventilation was assessed by calculating the PCD from the SF6 washout curves as described by Fowler et al. (20) , but with the modification suggested by Bouhuyset al. (21) , i.e. end-tidal concentration was used instead of mean expired concentration. PCD compares the average time a tracer gas molecule remains in the lungs (T1)-which is identical to the average time of clearing the lungs-with the average time (T2) a tracer gas molecule would remain in a uniformly ventilated ideal lung with the same FRC, series dead space, and ventilated with the same tidal volume. PCD was defined as: 100 ? (T1 – T2)/T1. An interactive computer program was used (22) . This approximated the curve as the sum of exponentially decaying functions: the computer plotted the washout curve semilogarithmically, and the user indicated how lines should be drawn to get the exponential curve components. The “peel-off” method was used,i.e. the slowest component was first identified and subtracted, and the plot was then redrawn to identify the remaining component(s). In all infants, an adequate approximation of the curve was achieved with two components. In normal infants, previously reported PCD values using N2 washout (8) were below 30%, in RDS 7-40%, and in chronic lung disease 90-350%. This agrees with values calculated from SF6 washout in healthy anesthetized children (25 ± 8%) (22) and in mechanically ventilated neonates with no or mild-to-moderate lung disease (43 ± 22%) (18) .
Steff Maître Glacier